Optical velocimetry systems or velocimeters are non-intrusive instruments for measuring velocities of particles, moving surfaces, etc. An optical velocimetry system creates fringes made of bright planes of light alternating with dark planes within a measurement volume through which particles entrained in fluids or on moving surfaces propagate. The light scattered from the particles that bisect the fringes is detected and measured to determine the velocity of the particles.
In a Laser Doppler Velocimetry (LDV) system, the fringes are created by intersecting two laser beams together to generate interference fringes in a measurement volume. While a body propagates through the generated interference fringes, scattered light is generated and the intensity variation of the scattered light is substantially independent of the propagation direction. Therefore, the propagation direction of the body through the interference fringes cannot be determined from the measured intensity of the scattered light. A commonly used technique to resolve directional ambiguity consists in inserting a Bragg cell along the optical path of a given one of the two laser beams in order to frequency shift the given laser beam. However, such a technique requires additional pieces of equipment such as the Bragg cell for example, which increases the cost for the LDV system in addition to requiring further optical alignment procedures which are time-consuming. In addition, such LDV systems require the use of coherent light, i.e. laser light, for generating the interference fringes. Therefore, this feature limits the choice of adequate light sources for generating the interference fringes, and non-coherent light sources cannot be used for example. Furthermore, such LDV systems may not be adequate for specific applications such as some medical applications, due to laser safety issues.
An alternative approach for creating fringes consists in imaging a bar pattern such as a Ronchi grating in a measurement volume. As for the LDV system, the generated fringes are stationary and the same problem of directional ambiguity exists. Furthermore, the fringe spacing of the fringes generated using a Ronchi grating system is fixed and may not be changed. In order to change the fringe spacing, a different Ronchi grating must be used, which increases the cost. Furthermore, changing the Ronchi gratings and aligning the optical system thereafter are time-consuming steps.
Furthermore, these two approaches tend to afford only limited versatility. For example, orienting the fringes generated by an LDV system or a Ronchi grating system requires additional machinery, which also increases the overall cost for such systems.
Therefore, there is a need for an improved (and more versatile) method and system for determining the velocity of body within a fluid.